I think you'd have no problem grabbing a jumpseat for part of the flight. The A340 has a spacious cockpit with comfy jumpseats. On our flight, the cockpit door remained open for most of the flight, even during final approach. I peaked my head in during the approach. The pilots didn't mind at all. In fact, the co-pilot invited a girl from Biz class to sit in the jumpseat during the approach.
